The invention relates to a low-pressure mercury vapour discharge lamp having a closed discharge vessel which contains mercury and rare gas and within which electrodes are positioned between which the discharge is maintained during operation, each electrode being formed by a tungsten coil supported at either end by a respective current supply wire, which coil has central turns covered with at least one emitter and, on either side thereof, end turns which are free from emitter.
Low-pressure mercury vapour discharge lamps of the kind described are well-known. Such lamps include, for example, fluorescent lamps constructed as straight tubes as, for example, disclosed in U.S. Pat. No. 3,937,998, and compact fluorescent lamps as, for example, disclosed in U.S. Pat. Nos. 4,374,340 and 4,546,285.
The emitter on the central turns of the tungsten coil provides a reduction of the emission potential of electrons emitted by the tungsten coil during operation of the lamp. A mixture of barium oxide, strontium oxide, and calcium oxide, for example, is used as an emitter. The barium therein is the main active ingredient, while the strontium and the calcium reduce the vapour pressure of the barium. During the manufacture of the lamp, triple carbonate (Ba-Sr-Ca carbonate) is provided on the central turns and converted into Ba-Sr-Ca oxide by heating through passage of electric current through the tungsten coil, whereby CO.sub.2 is evolved. The end turns and the current supply wire ends are not coated with emitter because the temperature of the end turns and the current supply wire ends remains too low during passage of current for a good conversion of the triple carbonate into oxides to be realised. This would mean that later, in the finished lamp, CO.sub.2 would be evolved during operation, which is disastrous for lamp life.
Low-pressure mercury vapour discharge lamps can be subdivided into so-called hot starting and cold starting lamps. In hot starting lamps, the tungsten coil is preheated before ignition of the lamp in that an electric current is passed through it. In cold starting lamps, the discharge is initiated by a glow discharge at he area of a tungsten coil, for example, between one tungsten electrode and an ignition strip which is connected to the other tungsten electrode, or between the two electrodes.
A problem with cold starting low-pressure mercury vapour discharge lamps is the switching life of the lamps. It has been found that the lamps reach the end of their lives already after a comparatively low number of on/off switching operations.